1. Field of Invention
The invention relates to high-quality, large-diameter silicon carbide (SiC) single crystals of 4H and 6H polytype and the sublimation growth process thereof. The SiC single crystals of invention can be used in semiconductor, electronic and optoelectronic devices, such as high power and high frequency diodes and transistors, ultra-fast semiconductor optical switches, detectors working in harsh environments and many others.
The invention is an improved process of SiC sublimation crystal growth. The main novel aspect of the invention is in control of the vapor transport and temperature gradients, wherein said transport is restricted to the central area of the growing crystal, while the crystal and its environs are under conditions of near-zero radial temperature gradients. This leads to the advantageously shaped growth interface, such as flat or slightly convex towards the source, reduced crystal stress and reduced densities of crystal defects.
Other novel aspects of the invention include in-situ densification of the SiC source by sublimation and filtration of the vapor from particulates originating from the SiC source. As an optional feature, the invention comprises a step of in-situ synthesis of the SiC source from elemental components.
The SiC single crystals grown by the growth process of invention are suitable for the fabrication of large-diameter, high-quality SiC single crystal substrates of 4H and 6H polytype, n-type and semi-insulating, including substrates of 100 mm, 125 mm, 150 mm and 200 mm in diameter.
2. Description of Related Art
Over the last decade, significant progress has been achieved in SiC crystal growth and substrate manufacturing. Currently, the largest SiC substrates available commercially are 4H and 6H SiC wafers of 100 mm in diameter. 150 mm substrates have been under development, and, recently, limited quantities of 150 mm n-type substrates became available on trial or sampling basis. Broad implementation of 150 mm diameter SiC substrates and, in the future, 200 mm substrates will enable significant cost reduction of SiC- and GaN-based semiconductor devices.
Development-grade 150 mm n-type wafers are known in the art. However, progress in SiC-based devices is still hampered by the scarcity of commercially available, high-quality 150 mm SiC substrates and by the absence of 200 mm substrates.
Harmful defects in SiC substrates include: dislocations, micropipes, stacking faults, inclusions of foreign polytypes and carbon inclusions. Stress in the SiC substrate is another factor detrimental to the device performance and technology.
Dislocations and Micropipes
The main dislocation types in hexagonal SiC are: Threading Screw Dislocations (TSD), Threading Edge Dislocations (TED) and Basal Plane Dislocations (BPD). The term ‘threading’ means that the dislocation line is approximately parallel to the hexagonal <0001> axis. The term ‘basal’ means that the dislocation line lies in the basal hexagonal (0001) plane. TSDs cause leakage and device degradation, while BPDs lead to the generation of stacking faults under bias and, subsequently, to the terminal device failure. TEDs are viewed as relatively benign defects.
Micropipes (MP) are hollow-core TSDs with the Burgers vector exceeding 3c, where c is the lattice parameter. 4H SiC homoepitaxial layers are commonly grown on 4° off-cut substrates, whereupon at least a fraction of dislocations and micropipes present extends into the epilayer from the substrate. MPs are “device killers” causing severe charge leakage even at low bias voltages.
Etching in KOH-based fluxes is commonly used to reveal etch pits due to dislocations and MPs—each dislocation type produces etch pits of characteristic geometry. In addition to etching, MP density (MPD) can be determined optically, by studying polished SiC wafers under a polarizing microscope. Upon etching on-axis SiC wafers (i.e. oriented parallel to the hexagonal c-plane) or wafers oriented several degrees off-axis, each threading dislocation and MP produces one etch pit on the wafer surface. Therefore, MP, TSD and TED densities are measured as the number of corresponding etch pits per 1 cm2 of the wafer surface.
The terms ‘dislocation density’, ‘total dislocation density’ and ‘wafer-average dislocation density’ used in the SiC related literature and in the present disclosure are understood as the density of etch pits per 1 cm2 of the wafer surface and, therefore, signify the density of threading dislocations.
BPD lines are in the basal plane, and the number of etch pits BPDs produce depends on the wafer off-cut angle. For instance, BPDs do not produce etch pits in on-axis wafers. The best way to reveal BPDs is by x-ray topography, where they are visible as a plurality of curved lines. Accordingly, the BPD density is calculated as the total length of the BPD lines (cm) per total analyzed volume of the substrate (cm3), i.e. in the units of cm/cm3.
Micropipes are hollow-core TSDs with the Burgers vector exceeding 3c, where c is the lattice parameter. Micropipes are stress concentrators triggering generation of BPD loops around the micropipe. Micropipes are “device killers” causing severe charge leakage even at low bias voltages. In addition to etching, micropipe density (MPD) can be determined optically, by studying polished SiC wafers under a polarizing microscope.
3″ and 100 mm SiC substrates with MPD=0 have been demonstrated by several commercial manufacturers. However, average MPD values in commercial substrates are, typically, higher than 0.1-0.2 cm2.
Stacking Faults
For 4H and 6H polytypes, the normal stacking sequences in the <0001> direction are ‘ABCB’ and ‘ABCACB’, respectively. Stacking faults (SFs) are two-dimensional defects violating the ideal stacking sequence and emerging as a result of non-optimized growth conditions. During homoepitaxial growth on 4° off-cut 4H SiC substrates, SFs propagate from the substrate into the epilayer. The presence of SFs can be detected by x-ray topography and photoluminescence. Based on the x-ray topography, the SF density can be expressed as percentage of the substrate area occupied by SFs. SFs are terminal for the devices.
Inclusions of Foreign Polytypes
Free energies of various SiC polytypes are close, and polytype inclusions, such as 15R, are frequently observed in 4H and 6H crystals, especially when the growth conditions are non-optimized or unstable. The lattice of 15R is rhombohedral, and 15R inclusions in hexagonal 4H and 6H lead to crude defects, such as dislocation walls and clusters of micropipes.
Carbon Inclusions
Carbon inclusions are common in SiC crystals, and their origin is usually assigned to the spent, carbonized SiC source. Evaporation of SiC is incongruent, with the vapor enriched with silicon. As a result, gradual accumulation of the carbon residue, which is a light and flaky substance, takes place during growth. Carbon particles from the residue become airborne and, transported by the vapor flux, incorporate into the growing crystal. Carbon inclusions, which can be from a fraction of a millimeter to several microns in size, are often visible in a polished wafer as light-scattering clouds. Large carbon inclusions lead to micropipes, while clouds of small-size inclusions increase the dislocation density.
X-Ray Quality
The method of x-ray rocking curves provides quantitative information on the lattice curvature and broadening of the x-ray reflection. The lattice curvature is expressed as ΔΩ, which is a variation of the sample angle Ω between different points on the wafer surface (ΔΩ=ΩMAX−ΩMIN). High values of ΔΩ are indicative of strong lattice deformation. In highest quality SiC substrates, ΔΩ is below 0.1°, while in present-day commercial SiC substrates, ΔΩ values as high as 0.2-0.3° are often observed.
X-ray reflection broadening is expressed as Full Width at Half Maximum (FWHM) of the reflection peak. High FWHM values are a consequence of lattice disorder, such as high density of dislocations and low-angle grains. For highest quality 4H SiC substrates, the value of FWHM is on the order of 10-12 arc-seconds and comparable to the angular divergence of the incident monochromatic x-ray beam. In the present-day commercial SiC wafers, the values of FWHM are, typically, above 15 arc-seconds and up to 75-100 arc-seconds. A FWHM value above 25-30 arc-seconds is a sign of inferior crystal quality.
Stress
In a SiC wafer, one can distinguish global, wafer-size stress and local stress. The magnitude of stress can be quantified by Raman spectroscopy or by special x-ray methods. However, a much simpler, qualitative technique is routinely applied to SiC wafers—visual inspection under crossed polarizers. Based on the cross-polarizer contrast, the level of stress and its uniformity can be assessed qualitatively, and various macroscopic defects, such as dislocation clusters, polytype inclusions, grain boundaries, etc. can be found. The cross-polarizer contrast is usually classified qualitatively as ‘low’, ‘medium’ or ‘high’.
SiC Sublimation Growth of Prior Art
The sublimation technique of Physical Vapor Transport (PVT) is widely used for the growth of commercial-size SiC single crystals. A conventional SiC sublimation growth cell of Prior Art is shown schematically in FIG. 1. The process is carried out in a gas-tight chamber 10, which is usually made of fused silica. The chamber 10 includes a growth crucible 11 and thermal insulation 12 which surrounds the crucible 11. The growth crucible 11 is generally made of dense, fine-grain graphite, while the thermal insulation 12 is made from lightweight, fibrous graphite. Most commonly, heating is provided by a single RF coil 16, which couples electromagnetically to the crucible 11. However, the use of resistive heating is envisioned.
The crucible 11 includes SiC sublimation source 14 and a SiC single crystal seed 15. Most commonly, the source 14 (polycrystalline SiC grain) is disposed at the bottom of the crucible 11, while the seed 15 at the top, for instance, is attached to crucible lid 11a.
At growth temperatures (typically, between 2000° C. and 2400° C.), the SiC source 14 vaporizes and fills the crucible with vapors of Si2C, SiC2 and Si molecules. During growth, the temperature of the source 14 is maintained higher than that of the seed 15, leading to temperature gradients in the growth crucible, both axial and radial, on the order of 10-30° C./cm. The vapors migrate to the seed 15 and precipitate on said seed causing growth of a SiC single crystal 17 on the seed 15. The vapor transport in the crucible is signified by arrows 19 in FIG. 1. In order to control the growth rate and ensure crystal quality, sublimation growth is carried out under a small pressure of inert gas, generally, between several and 100 Torr.
As one of ordinary skill in the art of SiC sublimation growth would recognize, two technological factors are crucial to the crystal quality: the magnitude of radial temperature gradients within the growing SiC single crystal 17 and the shape of the crystal growth interface 20. Steep radial gradients cause stress and appearance of stress-related crystal defects, such as BPDs. A strongly curved growth interface, convex or concave, leads to the appearance of crude macrosteps on the interface, stacking faults, inclusions of foreign polytypes and other defects. A concave toward the source (hereafter ‘concave’) interface leads to the generation and rapid accumulation of various defects during growth. It is generally believed that a flat or slightly convex toward the source (hereafter ‘convex’) growth interface is the most conducive to high crystal quality.
It is commonly believed that the growth interface follows closely the isotherm shape: concave isotherms result in a concave interface 20, while convex isotherms yield a convex interface 20. Radial temperature gradients are positive when the temperature increases in the radial direction from the crucible axis toward the crucible wall. Positive radial temperature gradients produce convex isotherms. Radial temperature gradients are negative when the temperature decreases in the radial direction from the crucible axis toward the crucible wall. Negative radial temperature gradients produce concave isotherms. A zero radial gradient produces flat isotherms.
The conventional, single-coil SiC sublimation growth arrangement from FIG. 1 suffers from poorly controllable radial temperature gradients, especially when the crystal diameter is large. With increase in the diameter of the crucible 11 and the RF coil 16, electromagnetic coupling between crucible 11 and RF coil 16 becomes less efficient, thermal fields less uniform and radial gradients steeper. A SiC sublimation growth method aimed at reduction of the harmful radial gradients is disclosed in U.S. Pat. No. 6,800,136 (hereinafter “the '136 patent”).
The SiC sublimation growth system disclosed in the '136 patent utilizes two independent flat heaters, namely, a source heater and a boule heater, which can be either inductive or resistive. The heaters are positioned coaxially with the crucible—the source heater is disposed below the source material, while the boule heater is disposed above the growing crystal. In order to reduce the radial heat losses, desirably to zero, the growth apparatus of the '136 patent comprises thick cylindrical thermal insulation with an option of an additional cylindrical heater disposed around the growth cell. Disadvantages of the growth system disclosed in the '136 patent include poor coupling of the flat coils to the cylindrical crucible, while disk-shaped resistive heaters obstruct heat dissipation in the axial direction, leading to strongly negative radial gradients.
An improved SiC sublimation growth method disclosed in US 2010/0139552 is shown in FIG. 2A. The growth apparatus includes cylindrical growth crucible 20 30 including SiC source material 21, a SiC seed 22 and a SiC single crystal 23 growing on the seed 22. The crucible 20 30 is positioned between two resistive heaters, top heater 28 and bottom heater 29, disposed coaxially with the crucible 20 30. The growth crucible 20 30 and heaters 28 and 29 are surrounded by thermal insulation (not shown).
The top heater 28 is ring-shaped with through hole 28a at the center. The bottom, cup-shaped heater 29 comprises two sections: ring-shaped section 29a with central hole 29b and cylindrically-shaped section 29c. The bottom heater 29 is disposed below and around the source material 21 included in the growth crucible 20 30.
FIG. 2B shows results of modeling of the growth cell from FIG. 2A. The isotherms 25 and the contour of a 3 inch diameter SiC crystal 23 were obtained by finite element simulation. The thermal field in the crucible 20 30 can be tuned by adjusting the current supplied to the heaters 29 and 28 to produce positive and shallow radial gradients within the crystal 23. Still, when this improved technique was applied to the growth of larger-diameter boules, such as 150 mm diameter boules, the growth interface was concave or wavy, such as concave at the center and convex at the periphery.
This is illustrated in FIG. 3, which depicts a growth cell similar to that of FIG. 2A, but scaled up for the growth of 150 mm crystals. The isotherms 35 and the contour of a 150 mm SiC crystal 33 growing on a SiC seed crystal 34 were obtained by finite element simulation. One can see that in spite of the isotherm convexity, the growth interface at the center of crystal 33 is concave.
A common practical approach to achieving the convex growth interface is in further increase of the isotherm convexity. However, strongly convex isotherms and associated with them steep radial gradients cause stress and crystal defects. It is postulated herein that the common perception of the growth interface following closely the shape of the isotherms is inaccurate, and that the interface shape is determined not only by the isotherms, but by the geometry of the vapor transport as well.
Generally, the temperature distribution within the SiC source material 31 is spatially nonuniform with the highest temperatures reached in the areas 36 adjacent to the crucible walls. During growth, the source vaporizes from these hotter areas 36, leaving carbon residue behind, while a denser SiC body 37 is formed in the colder top area of the source material 31. As a result, vapors from the source material 31 arrive predominantly at the periphery of the growing SiC crystal 33, as shown by arrows 34 34′ in FIG. 3. The vapor molecules adsorb on the growth interface and diffuse in the adsorbed state towards the colder center of the SiC single crystal 33 boule. With this geometry of vapor transport, the growth interface at the center of the growing SiC crystal 33 has a tendency to be concave, especially, when the boule diameter is large. Reference numbers 38 and 39 in FIG. 3 correspond to heaters 28 and 29, respectively, in FIGS. 2A and 2B.
The effect of the vapor transport geometry on the crystal shape is illustrated in FIGS. 4A and 4B, which show two 150 mm boule contours produced by finite element modeling of heat and mass transport. The thermal boundary conditions were chosen to produce zero radial gradients, that is, flat isotherms. In the case shown in FIG. 4A, the vapors were supplied to the SiC single crystal boule periphery, yielding a concave interface at the center of the boule. In the case shown in FIG. 4B the vapors were supplied to the SiC single crystal boule center, yielding a convex interface at the center of the boule.
Another common problem of SiC sublimation growth is carbon inclusions originating from the spent (carbonized) source.
While the prior art of SiC sublimation growth is broad and numerous, there is still a need for a process that can reproducibly yield high-quality SiC single crystals suitable for the fabrication of high-quality, large-diameter SiC substrates, such as 100 mm, 125 mm, 150 mm and 200 mm in diameter.